St. Thomas on the Trinity:
"There are other truths about God revealed to us in the teaching of the Christian religion, which were beyond the reach of the philosophers. Therse are truths about which we are insructed, in accord with the norm of Christian faith, in a way that transcends human perception.
The teaching is that although God is one and simple...God is Father, God is Son, and God is Holy Spirit. And these three are not three gods, but one God. We now turn to a consideration of this truth, so far as is possible to us."
As a mystery, we can never fully understand the Trinity. But as Frank Sheed liked to say, God wouldn't have revealed it if we couldn't understand at least something about it. And whatever light we can get about God is worth the effort. Just to know that God is a Trinity of persons that we can never fully understand is to know a lot more than we would without that knowledge.